This invention relates to a length off displacement measuring apparatus and, more particularly, to an apparatus for measuring a length of displacement between two relatively movable members and an outer or inner diameter of a cylindrical member.
When automating machine tools and various industrial machines, it is necessary to accurately detect the amount of relative displacement or distance between two relatively movable members, for example, a bed and a table of the machine tool.
Among prior art apparatus for measuring the amount of relative movement, that is the displacement between two relatively movable members, are included apparatus such as an Inductsyn wherein a scale and a slider are mounted on the opposing surfaces of two movable members, an exciting current is passed through the scale, and an electric signal induced in the slider is used to detect the amount of relative movement of the two members, and apparatus wherein the rotational angle of a motor for moving one member relative to the other is detected by a Resolver so as to detect the amount of relative movement of the two members.
According to the former apparatus, it is necessary to accurately mount a machined scale and a slider on the opposing surfaces of the relatively movable members. Furthermore, for the purpose of increasing the accuracy of measurement, it is necessary to precisely finish the scale with its pitch as small as possible. This not only requires a large amount of labor and time but also increases the cost of manufacturing. On the other hand the latter apparatus requires a complicated electrical processing circuit, and even when the rotational angle of the motor is measured accurately, measurement errors are caused due to errors of the clearances of the speed reduction mechanism and the feed mechanism driven by the motor.
In the past, the diameter, for example the outer diameter, of a large workpiece machined by a vertical lathe was measured by a large micrometer specially manufactured for this purpose or by placing blocks on the diametrically opposite positions of the workpiece and then measuring the distance between the blocks with an inside micrometer.
However, such large micrometer is heavy and difficult to handle, thus causing poor accuracy of measurement, and where the inside micrometer is used it is difficult not only to accurately position the blocks in contact with the workpiece but also to accurately connect the micrometer.
Besides these apparatus there is another diameter measuring apparatus wherein a table supporting a workpiece is rotated, and a roller of a known diameter is urged against the peripheral surface of the workpiece so as to calculate the diameter of the workpiece by using the number of revolutions of the roller. However, due to slip of the roller, the accuracy of measurement is degraded. Moreover, when the roller is pressed against the surface of the workpiece, the surface would be damaged when it is made of relatively soft material.
When using an inside micrometer for measuring an inner diameter, as the inner diameter increases the same problem occurs.